In recent years, electric powered vehicles such as a hybrid vehicle, an electric vehicle and the like that are equipped with an electric motor as a source of force driving the vehicles to cause them to travel have been improved to increase a capacity of a power storage mechanism for improved driving performance such as enhanced acceleration performance and increased continuously travelable distance. A structure having a plurality of power storage devices arranged in parallel has been proposed as a technique for increasing the capacity of the power storage mechanism.
For example, Japanese Patent Laying-Open No, 2003-209969 (Patent Document 1) discloses a power supply control system for a motor generating vehicle driving force, including a plurality of power supply stages each configured of a set of a low voltage battery and a converter. In particular, Patent Document 1 controls a current to limit it for each power supply stage individually to balance the batteries' respective states of charge.
Furthermore, Japanese Patent Laying-open No. 2003-134606 (Patent Document 2) discloses that a single battery and a step up/down converter are combined together and thus used to control an electric motor unit to drive it and that when the battery fails or the like, the step up/down converter has its internal switch steadily turned on to ensure that the electric motor unit supplies electric power to feed an auxiliary therewith.
Generally, when a hybrid vehicle, an electric vehicle or a similar electric powered vehicle travels, and is regeneratively braked, its electric motor generates electric power, which is used to charge a power storage device to achieve increased mileage.
Furthermore in recent years there has been proposed charging an electric powered vehicle from an external power supply while the vehicle is stopped and subsequently parked. Charging a vehicle from the external power supply is done for example at night, and requires a relatively long period of time, and whether the vehicle is charged efficiently may arise a concern.
In particular, a power supply system configuration converting electric power between a power storage device and an electric motor via a converter is configured to also charge each power storage device via the converter in charging the power storage device from the external power supply, and in doing so, the converter is required to operate efficiently.
In particular, a power supply system having a plurality of power storage devices and a plurality of converters arranged in parallel, as described in Patent Document 1, requires preventing the plurality of converters from causing a loss decreasing the efficiency of charge using external power supply. However, Patent Documents 1 and 2 are silent on how in such a power supply system as described above the converters should be controlled in charging the power storage devices, from an external power supply for a long period of time in particular.